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Root growth potential (RGP) is the ability of a tree seedling to initiate and elongate roots when placed into an environment favorable for root growth. The magnitude of RGP is often correlated with survival, and even growth, of the seedling following outplanting. RGP develops in seedlings during their tenure in the nursery or greenhouse. The most critical factors over which the grower can exercise control are date of lifting and duration of cold storage. Following planting, the expression of RGP, or actual root growth, is affected by soil temperature, soil moisture, and other factors.
RGP is measured by (1) placing seedlings into an environment, such as a warm greenhouse, which is favorable for root growth, (2) holding them for a standard period of time, then (3) removing them and assessing the amount of root growth which occurred. This procedure can be shortened by increasing temperature and by using subjective indices to quantify root growth.
Of the various tests devised to assess stock quality, RGP is perhaps the most reliable predictor of field performance. This is particularly true with species which exhibit a long period of winter dormancy and have a pronounced chilling requirement to release dormancy -- this includes probably all northwest conifers and many northeastern hardwood species.